The joining of ceramics to metals is of ever increasing importance. Metal-ceramic bonding has already received considerable attention for such varied applications as cutting tools and electronic tubes. More recently, metal-ceramic bonding has found utility in the fabrication of closure seals for electrochemical cells so as to render the cells fluid-tight.
Conventionally, metallurgical bonding of metal members to ceramic members first required the ceramic part to be precoated with a metal which usually involves the sintering of an oxide of a metal on the ceramic surface with a subsequent reduction of the surface layer to metal, or the sintering of a metal powder on the ceramic surface. The reason for metallizing the ceramic surface is to prepare the surface for brazing or soldering the plated metal to another metal to produce a strong fluid-tight assembly. Since sintering requires the use of high temperature environments, care has to be taken not to distort or otherwise destroy the physical properties of the ceramic member.
Once a fusible metal joint is formed between the metal-plated ceramic member and the metal member by soldering or brazing techniques, the exposed area of the ceramic member still has a layer of the plated metal. In certain applications this may be acceptable, but when the part is intended to be employed in electronic tubes or as seals for electrochemical cells, then the plated metal will have to be removed from selected areas of the ceramic part so as to destroy any electronic paths between the joined members which could cause electrical shorting. Conventionally, the unwanted plated metal areas are removed from the ceramic part prior to soldering or brazing by a grinding operation which is cumbersome to perform and expensive when small ceramic parts are involved. Additionally, when a small ceramic insulating annulus or ring is to be metallurgically bonded to a metal member at its outer peripheral area and to a second metal member at its inner circular area, when two brazing or soldering rings are required - one for each fusible metal joint to be formed. This type of metal-to-ceramic-to-metal bonding is usually required for metal/ceramic seals for use in electrochemical cells. Not only is the fabrication of these seal assemblies expensive, but due to the small size of the overall assemblies, it is difficult to insure the forming of good fusible metal joints on a continuous and reliable basis.
One object of the present invention is to provide a method for metallurgically bonding a metal-plated ceramic insulator to a metal member whereby the plated metal on selected areas of the ceramic part can be chemically removed.
Another object of the present invention is to provide a method for metallurgically bonding a ceramic insulator annulus to a metal member at its peripheral edge and a metal member at its inner disposed edge using only one fusible metal ring.
Another object of the present invention is to provide a method for producing ceramic/metal seals for electrochemical cells such as oxyhalide cells.
Another object of the present invention is to provide a method for producing ceramic/metal seals for electrochemical cells that is relatively inexpensive to perform and consistent in its reliability to produce fluid-tight fusible metal joints.